The present invention pertains to rare earth phosphor admixtures utilizing thulium-activated lanthanum or gadolinium oxyhalide phosphor material to increase the relative speed and resolution of an x-ray image compared with conventional phosphors as well as reduce the still serious crossover problem now experienced with conventional phosphors.
In recently issued U.S. Pat. No. 3,795,814, there is described and claimed lanthanum and gadolinium oxyhalide phosphors activated with thulium as efficient materials to convert x-radiation to visible light. Various image converter devices utilizing said luminescent materials are also described for conversion of the x-rays to blue emission. A particular x-ray intensifying screen is disclosed for use with photographic film which is sensitive to the "blue-near ultraviolet" radiation being emitted by said phosphors.
A more recently issued U.S. Pat. No. 4,070,583 discloses a different x-ray intensifying screen which can be subject to poor resolution and a blurred image produced from what is termed a "crossover" problem. Said x-ray screen construction utilizes a double emulsion photographic film with a pair of phosphor layers which are oriented so that light emitted from each phosphor layer can expose both emulsion layers. The crossover problem arises from light passage entirely through the next adjacent emulsion layer for exposure of the more remote emulsion layer. For the x-ray screen construction described therein, a blue-sensitive photographic film is employed together with a particular rare-earth oxyhalide phosphor material which is coactivated with terbium and thulium to reduce the crossover problem effects. The improvement is attributable primarily to a greater UV emission of said phosphor material since UV emission is absorbed to a higher degree than visible light by the silver halide particles in the next adjacent emulsion layer.
On the other hand, a more severe crossover problem is encountered when green sensitive photographic film is customarily employed with known La.sub.2 O.sub.2 S:Tb and Gd.sub.2 O.sub.2 S:Tb phosphor admixtures in this type x-ray screen construction. Such combination has been found to exhibit poorer resolution along with a blurred photographic image since the green emission from said phosphor admixtures is not absorbed by the next adjacent phosphor layer to the same degree as blue emission. Thus, while the known phosphor admixture is selected for light emission in a spectral region where the green sensitive photographic film is most responsive, such selection produces a more serious crossover problem.
The seriousness and extent of said crossover and image resolution or sharpness problems in x-ray screens utilizing a double emulsion type green sensitive photographic film is disclosed in U.S. Pat. No. 4,130,428 along with means said to lessen both undesirable effects. A green light emitting screen is therein described for use with a particular silver halide emulsion film along with screening and filtering dyes being mentioned that reduce the amount of light crossover from 59% to 44%. Said result is also therein compared with a 51% light crossover said to exist for blue light emitting calcium tungstate screens when used with a blue base commercial photographic film. The phosphor materials selected for use in said improved x-ray screens are various rare earth oxychloride and oxysulfide phosphors activated with various rare earth elements including terbium and thulium with a preferred phosphor being a mixture of yttrium oxysulfide activated with terbium or terbium and dysprosium that is mixed with gadolinium or lanthanum or lutetium oxysulfide activated with terbium or dysprosium.
Accordingly, it is an important object of the present invention to provide a rare earth phosphor admixture having better resolution capabilities than presently used phosphor materials for a particular x-ray screen construction employing green-sensitive photographic film.
It is another object of the present invention to provide further improved radiographic screens employing green-sensitive photographic film by means of having associated optical filtering media in said construction.
Still another important objective of the present invention is to provide a rare earth phosphor mixture having better image resolution capability and less crossover problem when used with blue-sensitive photographic film as well as green-sensitive photographic film.